Scope sub-third-parties per parent

Replace the many-to-many junction table with a direct
parent_third_party_id foreign key on third_parties. Each
sub-third-party now belongs to exactly one parent, making
duplicates across parents independent entities.

Replace the firstLevel boolean with an integer level field
(1 = direct, 2+ = parent level + 1) to support arbitrary
nesting depth.

Remove the createThirdPartyThirdPartyMapping and
deleteThirdPartyThirdPartyMapping mutations, the CLI
link/unlink commands, and the corresponding MCP tools.
Creating a child third party now just requires passing
parentThirdPartyId on the existing createThirdParty mutation.

The frontend walks the parentThirdParty chain to build
display names like "Name (Ancestor1/Ancestor2)" and shows
clickable ancestor links on the detail page.

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/cmd/thirdpartymgmt/thirdpartymgmt.go

@@ -19,10 +19,8 @@ import (
 	"go.probo.inc/probo/pkg/cmd/cmdutil"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/create"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/delete"
-	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/link"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/list"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/publish"
-	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/unlink"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/update"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/vet"
 	"go.probo.inc/probo/pkg/cmd/thirdpartymgmt/view"
@@ -41,8 +39,6 @@ func NewCmdThirdParty(f *cmdutil.Factory) *cobra.Command {
 	cmd.AddCommand(delete.NewCmdDelete(f))
 	cmd.AddCommand(vet.NewCmdVet(f))
 	cmd.AddCommand(publish.NewCmdPublish(f))
-	cmd.AddCommand(link.NewCmdLink(f))
-	cmd.AddCommand(unlink.NewCmdUnlink(f))
 
 	return cmd
 }